Background Art
Metal ducts and tubes are routinely incorporated into numerous applications such as automotive vehicles, refrigeration systems and aircraft. Many of these metal ducts and tubes include irregular bends, bulges and/or indentations which can be problematic to form. Ducts and tubes that are relatively straight and that do not have harsh or abrupt bulges or indentations are commonly shaped using conventional bulge forming methods, including hydroforming.
A conventional bulge-forming device consists of an upper platen and a lower platen. A jig collar holds two matching die halves together around a tubular workpiece. A pair of pistons hold the tubular workpiece firmly between the die halves, an incompressible fluid is fed through one of the pistons and air is evacuated from the tubular workpiece from the other piston. When all of the air has been evacuated from the workpiece, a valve is closed permitting pressure to build up within the workpiece and causing the workpiece to bulge to match the contour of the die halves.
Conventional bulge forming has several limitations, the most notable of which pertains to its cost. Conventional bulge forming requires a press with relatively high tonnage and high strength tools that will withstand the application of hydraulic pressures of 20,000 p.s.i. or higher. In relatively high volume applications, conventional bulge forming may be a cost-effective alternative to other processes which tend to be more labor intensive. However, in relatively low volume applications, such as commercial aircraft, where only a couple hundred parts may need to be fabricated from a tool, the high cost of the presses and tooling associated with conventional bulge forming are prohibitive.
Other drawbacks of conventional bulge forming methods concern the geometry of the workpiece and the subsequent processing of the formed workpieces. Since pressurized fluid is applied against the interior of the entire workpiece, it remains a practical requirement that the workpiece be relatively straight and/or flat so as to simplify the geometry of the forming dies. Furthermore, it also remains a practical requirement that the workpiece be relatively short so as to avoid problems, such as pump capacity and cycle time, that are typically encountered when applying a high pressure fluid to a relatively large cavity.
One alternative to conventional bulge forming that has been used in low-volume applications has been to form small segments of the tube by hand and welding the segments together. While tooling costs for this method are relatively low, this process is extremely labor intensive and it is rather difficult to control the final quality of the tube or duct.
In commonly assigned U.S. Pat. No. 5,419,171 to Bumgarner, the disclosure of which is hereby incorporated by reference as if fully set forth herein, an improved isostatic bulge forming device and method is disclosed for forming a meal tube. The bulge-forming device employs a fluid pressure chamber having a valved inlet and a valved outlet for entry and egress of a forming fluid. The apparatus also includes a pair of mated tool halves that are retained in a fixturing tube and that collectively define a forming cavity. After the tool halves are sealingly engaged to a tubular workpiece and the workpiece is inserted into the tool, a pair of annular caps are placed in a fluid tight seal with the fixturing tube and the chamber is filled with an incompressible fluid. As pressurized fluid is permitted to travel to the interior of the tubular workpiece but not between the tool halves and the tubular workpiece, the pressurized fluid deforms the tubular workpiece to conform to the forming cavity.
While this method represents a significant advancement in the art for the forming of tubes and ducts on a low-volume basis, several drawbacks have been noted. Like conventional bulge forming methods, these drawbacks concern the geometry of the workpiece. Since pressurized fluid is applied against the interior of the entire workpiece, it remains a practical requirement that the workpiece be relatively straight and/or flat so as to simplify the geometry of the tool halves. Furthermore, it also remains a practical requirement that the workpiece be relatively short so as to avoid problems, such as pump capacity and cycle time, that are typically encountered when applying a high pressure fluid to a relatively large cavity.
Therefore, it would be desirable to provide a forming device for expanding tubular workpieces that provides a high quality formed tube at a relatively low-cost. It would also be highly desirable to provide a method for forming tubular workpieces that is cost-effective for relatively low-volume applications.